Various embodiments relate to a method for operating high-pressure discharge lamps, which method can be used e.g. in video projection systems, and operates the high-pressure discharge lamp with a variable power in order to minimize the energy consumption of the system and to increase the contrast ratio e.g. of the image to be reproduced. Primarily during the reproduction of film material, the image brightness is often very low (e.g. night scenes). In order to increase the dynamic contrast and in order to save energy and also in order to increase the average lamp lifetime by means of operation at on average lower power, therefore, the lamp is in this case intended to be adapted in terms of its power dynamically with the image content image by image. Although this is possible in principle, there is a problem here: if the momentary modulation of the lamp power is too great and the electrode temperature is thus subject to rapid fluctuations, thermal stresses between core pin and electrode filaments give rise to an “uncoiling” of the electrode filaments in the direction of the center of the arc. This leads very rapidly to a reduction of the electrode spacing, which then, as a result of the current limiting usually provided in the operating devices, has the effect that the lamps are operated with an excessively low power and can then also no longer be operated in the normal operating range. This considerably reduces the lifetime of the high-pressure discharge lamp. In addition, the image reproduction of the projection system is disturbed.
One solution is to greatly restrict the permitted modulation range, such that the “uncoiling” of the electrode filaments no longer occurs, or occurs so slowly that it is compensated for by the typical burn-back of the electrodes during the operating period.
Alternatively, only a very slow modulation could be permitted, such that there are only few momentary changes in power, but then a larger range of the modulation depth could be used. However, both solutions greatly restrict the contrast ratio obtainable by the modulation of the power, which is highly undesirable.